1. Field of Invention
The present invention relates generally to medical devices, and more particularly to therapeutic intravascular devices for reducing hypertension.
2. State of the Art
Hypertension (HTN) or high blood pressure, sometimes arterial hypertension, is a chronic medical condition in which the blood pressure in the arteries is elevated. This requires the heart to work harder than normal to circulate blood through the blood vessels. Blood pressure involves two measurements, systolic and diastolic, which depend on whether the heart muscle is contracting (systole) or relaxed (diastole) between beats. Normal blood pressure is at or below 120/80 mmHg. High blood pressure is said to be present if it is persistently at or above 140/90 mmHg.
Hypertension is classified as either primary (essential) hypertension or secondary hypertension; about 90-95% of cases are categorized as “primary hypertension” which means high blood pressure with no obvious underlying medical cause. The remaining 5-10% of cases (secondary hypertension) are caused by other conditions that affect the kidneys, arteries, heart or endocrine system.
Persistent hypertension is a major risk factor for stroke, myocardial infarction (heart attacks), heart failure and aneurysms of the arteries (e.g. aortic aneurysm), and is a cause of chronic kidney disease. Even moderate elevation of arterial blood pressure is associated with a shortened life expectancy.
Dietary and lifestyle changes can improve blood pressure control and decrease the risk of associated health complications, although drug treatment is often necessary in patients for whom lifestyle changes prove ineffective or insufficient. Several classes of medications, collectively referred to as antihypertensive drugs, are currently available for treating hypertension. However, some patients have proven resistant to reduced hypertension from treatment with drugs, or otherwise are not suitable candidates for such treatment.
It has been suggested that the neuromodulation of nerve fibers adjacent to renal arteries can substantially reduce systolic and diastolic blood pressure. US Pub. No. 2009/0076409 to Wu et al. describes a method of thermally-induced renal denervation (RDN) in which a device is inserted into the renal arteries, expanded into contact with the vessel wall, and operated to induce thermal damage to the renal nerve. Wu et al. states that treatment may require from seconds, to minutes, to days. Although there is some evidence that the device can achieve desired results, it can cause damage to the renal artery and other non-target tissue, and is subject to widely variable outcomes depending upon operator skill. Wu et al. also teaches that the blood flow through the vessel can be obstructed with a balloon, and the heat removed via injection of a thermal fluid. Obstruction of the renal vessel can damage downstream tissues and organs. As yet another alternative, Wu et al. teaches that the blood flow can be permitted continue through the renal vessel, and a thermal fluid can be injected to supplement heat transfer. However, there is no specific coordination between thermal fluid injection and neuromodulation; this can result in too much thermal fluid being administered or the thermal fluid being administered at a non-optimal time in the treatment.